Electrical energy generation relies on a variety of energy sources that are then converted into electricity, such as fossil fuel, nuclear, solar, wind, geothermic and hydroelectric. Apart from the concern of the dwindling supply of fossil fuel, one of the great challenges of energy supply chains is balancing supply with demand. As the supply of electricity turns to renewable energy, the latter concern becomes more acute as highest energy output may not match the highest energy demand. Thus, storing electrical energy in an efficient way becomes crucial to make these renewable energy sources more viable alternatives to fossil fuel.
All-liquid electrochemical cells offer viable solutions for the storage of energy in an uninterruptible power supply environment. These cells provide efficient storage capabilities because of the rapid ionic migration and facile, reversible reaction kinetics at both metallic electrodes. Energy is stored at the anode which is constituted mainly of a metal, referred to herein as the active metal or anodic metal, having a high chemical potential. In a discharged state, the active metal resides in the cathode at a low chemical potential in the form of an alloy. An electrolyte disposed between two molten electronically conductive electrodes and containing a cation of an anodic metal, enables ionic transport of the active metal during charging or discharging. For example, descriptions of such cells may be found in U.S. Pat. No. 8,323,816, US Patent Publication No. US-2011-0014505-A1, and US Patent Publication No. US-2012-0104990-A1, the entire contents of which are incorporated herein by reference.
Certain electrochemical cells which use bimetallic alloy at the cathode provide cells with a bigger thermodynamic differential because of the lowering of the chemical potential in the alloy compared to the individual metals. Alloying the cathode also provides the advantage of lowering the melting point of the cathode. For example, antimony melts at 631° C. and lead melts at 307° C., while the lead-antimony eutectic alloy melts at 253° C. This, in turn, allows the cells to operate at a lower temperature and/or operate more efficiently. Lowering the melting point of the cathode materials can also increase the solubility limit of the active metal in the cathode, which represents an increase in capacity of the cathode and decrease the cost per energy storage capacity of the cells. Furthermore, cells operating at a lower temperature should experience less corrosion and potentially extended operating lifespan of the cell. For example, descriptions of such cells may be found in US Patent Publication No. US-2012-0104990-A1, the entire content of which is incorporated herein by reference.
Typically, these electrochemical cells are assembled starting with solid state materials. Once all the materials are disposed into the housing, a cell is heated until the materials are melted. Because of the disparity in the density of each of the materials, the anodic metal, the electrolyte and the cathodic metals, upon melting, self-assemble into layers. The anodic and cathodic metals react to form an alloy resulting in the assembly of the cell in the discharged state.
Because of the great disparity of the chemical potential energy between the anodic and cathodic metals, the alloying reaction is very exothermic and uncontrollable and often leads to the formation of various intermetallic compounds of high melting points forming a heterogeneous solid phase. The overall morphology of this solid phase is difficult to control and very difficult to predict as it is the result of a violent chemical reaction involving release of large amounts of energy in a short duration of time. Internal shorting of the cell also often occurs, facilitated by the uncontrolled formation of this heterogeneous solid phase if it is able to establish electrical contact between the two current collectors. This often results in cell failure.